Electrically conductive compositions

ABSTRACT

Compositions comprising a polymer and dendrite crystals of a salt consisting of the cation of 2-(4,5-dihydronaphtho[1,2-d]-1,3-dithiole-2-ylidene)-4,5-dihydronaphtho [1,2-d]-1,3-dithiole and an anion are useful in forming conductive coatings.

This is a division of application Ser. No. 341,418, filed Jan. 21, 1982now U.S. Pat. No. 4,439,505.

FIELD OF THE INVENTION

The present invention relates to novel conductive compositions andelements. A novel process for making such compositions is alsodisclosed.

BACKGROUND OF THE INVENTION

The unwanted buildup of static electricity on an insulated support is acontinuing problem. It is well known that a thin conductive layer willprevent static buildup and it is possible to formulate a conductivecomposition that can be coated on a support. However, it has been quitedifficult to combine these conductive properties with other desirablephysical properties such as physical stability.

A number of charge transfer complexes are electrically conducting. Forexample, complex salts of2-(4,5-dihydronaphtho[1,2-d]-1,3-dithiol-2-ylidene)-4,5-dihydronaphtho[1,2-d]-1,3-dithiole(DTTF) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) are electricallyconducting. However, when such complexes are combined with anelectrically insulating polymer composition to incorporate otherphysical properties, the resulting composition exhibits no usefulelectrical conductivity. As a result, charge transfer complexes havebeen limited in their utility in forming useful antistatic andconductive layers for elements such as electrographic,electrophotographic and photographic elements.

It is desirable to obtain coating compositions of such charge transfercomplexes with electrically insulating polymers as those polymers wouldprovide physical stability for the coating, as well as other desirableproperties required for many applications.

SUMMARY OF THE INVENTION

The present invention provides a composition comprising a polymer whichis preferably organic solvent-soluble and a charge transfer complexcharacterized in that the charge transfer complex is (a) in the form ofdendrite crystals throughout the polymer and is (b) a salt consisting ofa cation of2-(4,5-dihydronaptho[1,2-d]-1,3-dithiol-2-ylidene)-4,5-dihydronaphtho-[1,2-d]-1,3-dithioleand an anion selected from the group consisting of7,7,8,8-tetracyanoquinodimethane (TCNQ⁻), ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, F⁻, Cl⁻,I⁻, and I₃ ⁻.

The compound2-(4,5-dihydronaphtho[1,2-d]-1,3-dithiol-2-ylidene)-4,5-dihydronaphtho[1,2-d]-1,3-dithiole,thereinafter referred to as DTTF, has the structure ##STR1##

The compositions have resistivities in the range from 10⁴ to 2×10⁸ohm/sq. depending on the particular organic-solvent-soluble polymericbinder used. The compositions are useful in the form of self-supportinglayers or as a coating on a support in any element in which it isdesirable to both provide stability and to avoid the buildup of staticelectricity or to provide an electrically conductive layer. The dendritecrystals, in combination with an electrically insulating polymer resultsin an electrically conducting composition.

The present invention also provides a method for making the novelelectrically conductive composition of the invention. The methodcomprises the steps of:

(a) forming a charge transfer complex from a cation of DTTF and an anionselected from the group consisting of TCNQ⁻, ClO₄ ⁻, BF₄, PF₆, F⁻, Cl⁻,I⁻ and I₃ ⁻ ;

(b) mixing from about 1 to 20 parts of the charge transfer complex withabout 5 to 200 parts of an electrically insulating polymer; and

(c) aggregating the charged transfer complex in the polymer therebyforming dendrite crystals throughout the polymer.

The preferred charge transfer complexes for the present inventioncomprise DTTF as the cation and an anion selected from the groupconsisting of TCNQ⁻ and ClO₄ ⁻.

DETAILED EMBODIMENTS OF THE INVENTION

The electrically conductive composition of this invention is formed bycombining the charge transfer complex with an electrically insulatingpolymeric binder in an organic solvent for the polymer and then coatingthe resulting composition on a suitable support and drying. Theinsulating coating has a resistance greater than 2×10⁸ ohms/sq.

The coating composition is rendered conductive by aggregating the chargetransfer complex in the polymer. One technique for aggregating thecharge transfer complex in the polymer is by contacting the compositionof the charge transfer complex and polymer with vapors of a solventwhich is capable of being absorbed into (penetrating) the layers. Suchvapor exposure is generally effective to induce aggregation at about 70°F. after about two minutes. Likewise, inhibition of solvent removal inan otherwise conventional coating of a dope solution comprising the dyeand polymer results in aggregation. Immersing the coating in a solvent,or coating from an original solvent mixture which contains a highboiling solvent which persists in the coating during drying, are amongother methods of inducing aggregation of the charge transfer complex.The aggregated coating is characterized by dendrite crystals throughoutthe polymeric binder. Useful organic solvents include chloroform,toluene, dichloromethane, acetone, acetonitrile, tetrahydrofuran,p-dioxane and trichloropropane.

Examples of useful polymers are selected from polycarbonates,polyesters, polysulfones, polyacrylates, polymethacrylates,poly(vinylbutyrals), poly(vinylalcohol) and polyacetals. Specificexamples of useful polymers include poly(4,4'-isopropylidenediphenylenecarbonate),poly(oxy-1,4-phenylenesulfonyl-1,4-phenyleneoxy-1,4-phenyleneisopropylidene-1,4-phenylene),poly[ethylene-co-isopropylidene-bis-(1,4-phenyleneoxy-ethylene)-terephthalate],poly(vinylbutyral), and poly(n-butylmethacrylate-costyrenesulfonic acid,potassium salt.

In general, the amount of the charge transfer complex combined with theelectrically insulating polymer to produce the electrically conductingcompositions in layers of the present invention varies widely. All thatis required is that enough of the charge transfer complex is used toform the network of dendrite crystals throughout the polymer. Ingeneral, the charge transfer complex is present in the electricallyinsulating polymer in an amount of about 0.1% to 20 weight percent basedon the total weight of the composition.

The compositions of the invention are usefully coated on a wide varietyof supports to form useful antistatic or conducting elements. Forexample, the coating compositions described herein can be coated onpolymeric materials such as poly(ethyleneterephthalate), celluloseacetate, polystyrene, poly(methyl methacrylate) and the like as well asother supports such as glass, paper including resin-coated paper andmetals. Fibers, including synthetic fibers, useful for weaving intocloth, are also examples of useful supports. Planar supports such aspolymeric films are particularly useful for photographic elements. Thecompositions of the present invention are useful in virtually anyarticle where it is desired to have a conductive coating. For example,the compositions are coated on small plastic parts to prevent theunwanted buildup of static electricity or coated on small polymericspheres or other shapes such as those used for toners in electrographyand the like.

The compositions of the present invention are coated onto the supportusing any suitable method. For example, the compositions are coated byspray coating, fluidized bed coating, dip coating, doctor blade coatingor extrusion hopper coating, to mention a few well-known coatingtechniques.

In some embodiments, it is desirable to overcoat the layer of thecompositions of the present invention with a protective layer. Theprotective layer is present for a variety of reasons. For example, theprotective layer is an abrasion-resistant layer or a layer whichprovides other desirable physical properties. In many embodiments, forexample, it is desirable to protect the conductive layers of the presentinvention from conditions which could adversely affect the aggregatedcomposition. The protective layer is generally a film-forming polymerwhich is applied using coating techniques such as those described abovefor forming the conductive layer itself.

The compositions of the present invention are particularly useful informing antistatic layers for photographic elements or conductive layersin electrographic and electrophotographic elements. These elementscomprise a support having coated thereon at least oneradiation-sensitive layer.

While the conductive layers described herein can be located anywhere ina photographic or electrophotographic element, it is preferred that theconductive layer be coated on the side of the support opposite the sidehaving the coating of the radiation-sensitive material. The coatingcompositions of the present invention are advantageously coated directlyon the support which has a thin subbing layer and are then overcoatedwith the described protective layer. Alternatively, the conductivelayers of the present invention are on the same side of the support asthe radiation-sensitive materials and the protective layers are includedas interlayers or overcoats, if desired.

The radiation-sensitive layers of the photographic orelectrophotographic elements of the present invention can comprisephotographic silver salt emulsions, such as silver halide emulsions;diazo-type compositions; vesicular image-forming compositions;photopolymerizable compositions; electrophotographic compositionscomprising radiation-sensitive photoconductors and the like.Photographic silver halide emulsions are particularly preferred and aredescribed, for example, in Product Licensing Index, Publication 9232,Vol. 92, December, 1971, pages 107-110.

A particularly useful element of the present invention is anelectrographic element. The conductive layers of the present invention,because of the uniformity of their conductivity and the humidityindependence of their conductivity, are excellent conductive layers forsuch an element. This embodiment comprises a support having coatedthereon the conductive layer as described herein and, as the outermostlayer, a dielectric layer. The dielectric layer is formed from anydielectric film-forming material. Examples of such materials include anyof the electrically insulating polymers used in forming the compositionsand layers of the invention and the polymers listed above as useful asthe protective layer.

Electrographic elements including electrophotographic elements, are wellknown in the art and are described, for example, by Dessauer and Clark,Xerography and Related Processes, Focal Press, 1965, Chapter XVI, pages439-450.

The resistance of the surface of the coatings of the present inventionis measured using well-known techniques. The resistivity is theelectrical resistance of a square of a thin film of material measured inthe plane of the material between opposite sides. This is described morefully in R. E. Atchison, Aust. J. Appl. Sci., 10 (1954).

The salts of the invention were prepared according to the followingprocedures:

PREPARATION I Preparation of DTFF:TCNQ Salts ##STR2##

A solution was made by dissolving 7.2 g (0.025 mole) of4,5-(3,4-dihydronaphtho[a])-1,3-dithioliumtetrafluoroborate in 55 ml ofacetonitrile containing 7.59 g (0.075 mole) of triethylamine by addingthe latter dropwise with magnetic stirring over 15 minutes. Another 15ml of acetonitrile was added and the mixture stirred for one hour atambient temperature. As the solution cooled in an ice bath, orangecrystals precipitated. They were collected by filtration. After washingwith cold acetonitrile, the product was air-dried. A yield of 4.32 g(85%) of DTFF as an orange solid was obtained.

A solution of 0.126 mmoles of DTFF was dissolved in hot tetrahydrofuran(THF). 0.176 mmoles of TCNQ was dissolved in hot acetonitrile and addedto the DTFF solution. The mixture was allowed to cool. The DTFF:TCNQsalt was obtained as a grey-black precipitate.

PREPARATION II Preparation of DTFF:ClO₄ Salts

To a suspension of 1.00 mmole of DTFF in 60 ml of hot acetonitrile wasadded 1.007 mmoles of perchloric acid in acetonitrile and 0.504 mmole ofH₂ O₂ in water. The solution was concentrated to 1/3 of its originalvolume and then cooled to room temperature. A DTFF:ClO₄ of mixedcomposition was obtained as a green crystalline precipitate. Theprecipitate was filtered and washed with cold acetonitrile.

The above procedure was used to prepare three separate compositionshaving the following stoichiometries:

(a) C₂₂ H₁₆ S₃.6 (ClO₄)₀.32 +O₀.76

(b) C₂₂ H₁₈ S₃.6 (ClO₄)₀.35 +O₀.71

(c) C₂₂ H₁₆ S₃.6 (ClO₄)₀.32 +O₁.22

The following examples illustrate the conductivity achieved by thecompositions of the present invention.

EXAMPLE 1 Preparation and Electrical Properties ofpoly(4,4'-isopropylidenediphenylene carbonate) Films ContainingDTFF:TCNQ Salt

To a solution of 5 mg of DTFF:TCNQ in 1:1 THF:acetonitrile (by volume)was added 2 ml of a solution containing 100 mg ofpoly(4,4'-isopropylidenediphenylene carbonate) in 1:1dichloromethane:p-dioxane. The solution was coated on unsubbed polyestersupport to a wet thickness 0.002 mil, and dried in air. Samples of thedried film were vapor-treated with various solvents for varying lengthsof time and the resistance was measured on a 2 cm by 2 cm square sampleusing silver paste contacts. The results are disclosed in Table I.

                  TABLE I                                                         ______________________________________                                                                     Resistance                                                     Vapor Treatment                                                                              of Films                                         Solvent       Time (in seconds)                                                                            (Ohm/Square)                                     ______________________________________                                        Untreated Film               >10.sup.9                                        Chloroform    25             2.8 × 10.sup.5                             Dichloromethane                                                                             25             5.1 × 10.sup.5                             Acetone       35             2.8 × 10.sup.5                             Acetonitrile  20             10 × 10.sup.5                              THF (tetrahydrofuran)                                                                       15             80 × 10.sup.5                              THF           35             5.9 × 10.sup.5                             ______________________________________                                    

EXAMPLE 2 Polysulfone Films Containing DTFF:TCNQ Salt

A coating solution was made by dissolving 8.7 mg of DTFF:TCNQ in 1.74 mlof dioxane containing 174 mg of a polysulfone having the structure##STR3## The coatings were dried and vapor-treated as described inExample 2. Resistance values are disclosed in Table II.

                  TABLE II                                                        ______________________________________                                                                   Resistance                                                       Vapor Treatment                                                                            of Films                                           Solvent       Time (in seconds)                                                                          (Ohm/Square)                                       ______________________________________                                        Untreated Film             >10.sup.9                                          Chloroform    20             9 × 10.sup.4                               Toluene       30            11 × 10.sup.4                               Trichloropropane                                                                            30           1.6 × 10.sup.4                               Trichloropropane                                                                            90           2.0 × 10.sup.4                               ______________________________________                                    

EXAMPLE 3Poly[ethylene-co-isopropylidenebis-(1,4-phenylene-oxyethylene)-terephthalate]Films Containing DTFF:TCNQ Salt

A mixture of 0.623 mmoles of DTFF was dissolved in hot THF, and 0.686mmoles of TCNQ dissolved in hot acetonitrile was concentrated from 75 mlto 20 ml by heating. On cooling, a 1:1 salt of DTFF:TCNQ was obtained asa grey-black product.

This salt was coated inPoly[ethylene-co-isopropylidenebis-(1,4-phenyleneoxy-ethylene)-terephthalate]as in Example 2 and then vapor-treated with chloroform for 120 seconds.The resistivity was 2.2×10⁴ ohm/square. Microscopic examination of thisfilm showed a multiplicity of connected dendrites or filament-likecrystals of DTFF:TCNQ throughout the polyester.

EXAMPLE 4 Electrical Properties of the DTFF:ClO₄ Salts in PolymericFilms

A coating was made from a solution of 8.7 mg of DTFF:ClO₄ prepared as inExample 5, dissolved in hot dichloromethane with 174 mg of differentpolymers in 1.7 ml of dichloromethane. The ratio of salt to polymer was1:20 by weight. The solution was coated with a wet thickness of 3 mils,dried, and vapor-treated with several solvents. The resistance ofDTTF:ClO₄ mils in poly[ethylene-co-ethylene)-terephthalate] films aredescribed in Table III.

                  TABLE III                                                       ______________________________________                                                    Vapor Treatment                                                                            Resistance                                           Solvent     Time (in seconds)                                                                          (Ohm/Square)                                         ______________________________________                                        Acetonitrile                                                                              35           1.8 × 10.sup.6                                 Dioxane     120          1.2 × 10.sup.7                                 Chloroform  60           1.15 × 10.sup.8                                ______________________________________                                    

The resistance of DTTF:ClO₄ salt in poly(4,4-isopropylidenediphenylenecarbonate) films are disclosed in Table IV.

                  TABLE IV                                                        ______________________________________                                                      Vapor Treatment                                                                            Resistance                                         Solvent       Time (in seconds)                                                                          (Ohm/Square)                                       ______________________________________                                        Acetonitrile   60          2.6 × 10.sup.6                               Acetonitrile  180          1.2 × 10.sup.6                               Trichloropropane                                                                             90            4 × 10.sup.6                               Trichloropropane                                                                            360          8.4 × 10.sup.4                               ______________________________________                                    

The resistance of DTTF:ClO₄ salts in poly(vinylbutyral) films aredisclosed in Table V.

                  TABLE V                                                         ______________________________________                                                      Vapor Treatment                                                                            Resistance                                         Solvent       Time (in seconds)                                                                          (Ohm/Square)                                       ______________________________________                                        Acetonitrile  60           2.6 × 10.sup.6                               Trichloropropane                                                                            60           1.1 × 10.sup.7                               Chloroform    60           6.6 × 10.sup.6                               Dioxane       60             7 × 10.sup.6                               Trichloropropane                                                                            120          4.2 × 10.sup.6                               Poly(n-butylmethacrylate and p-styrene-sulfonic acid                          potassium salt)                                                               ______________________________________                                    

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. An electrographic element having a conductivelayer comprising an electrically insulating polymer and a chargetransfer complex characterized in that the charge transfer complex is(a) in the form of dendrite crystals throughout the polymer and (b) is asalt consisting of a cation of2-(4,5-dihydronaphtho[1,2-d]-1,3-dithiol-2-ylidene)-4,5-dihydronaphtho[1,2-d]-1,3-dithioleand an anion selected from the group consisting of7,7,8,8-tetracyanoquinodimethane, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, F⁻, Cl⁻, I⁻, andI₃ ⁻.
 2. A photographic element having an antistatic layer comprising anelectrically insulating polymer and a charge transfer complexcharacterized in that the charge transfer complex is (a) in the form ofdendrite crystals throughout the polymer and (b) is a salt consisting ofa cation of2-(4,5-dihydronaphtho[1,2-d]-1,3-dithiol-2-ylidene)-4,5-dihydronaphtho[1,2-d]-1,3-dithioleand an anion selected from the group consisting of 7,7,8,8⁻tetracyanoquinodimethane, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, F⁻, Cl⁻, I⁻, and I₃ ⁻.3. An electrophotographic element having a conductive layer comprisingan electrically insulating polymer and a charge transfer complexcharacterized in that the charge transfer complex is (a) in the formdendrite crystals throughout the polymer and (b) is a salt consisting ofa cation of2-(4,5-dihydronaphtho[1,2-d]-1,3-dithiol-2-ylidene)-4,5-dihydronaphtho[1,2-d]-1,3-dithioleand an anion selected from the group consisting of 7,7,8,8⁻tetracyanoquinodimethane, ClO₄ ⁻, BF₄ ⁻, PF₆ ⁻, F⁻, Cl⁻, I⁻, and I₃ ⁻.4. An element as in claims 1, 2 or 3 in which the anion is selected fromthe group consisting of 7,7,8,8⁻ tetracyanoquinodimethane and ClO₄ ⁻. 5.An element as in claims 1, 2 or 3 in which the electrically insulatingpolymer is selected from the group consisting of polycarbonates,polyesters and polysulfones.
 6. An element is in claims 1, 2 or 3 inwhich the electrically insulating polymer is selected from the groupconsisting of poly(4,4'-isopropylidenediphenylene carbonate),poly(oxy-1,4-phenylenesulfonyl-1,4-phenyleneoxy-1,4-phenyleneisopropylidene-1,4-phenylene),poly[ethylene-co-isopropylidenebis-(1,4-phenyleneoxy-ethylene)-terephthalate],poly(vinyl butyral), and poly(n-butylmethacrylate-co-p-styrenesulfonicacid potassium salt).